The Torah commands in Parashat Mishpatim (Shemot 23:18), “Ve’lo Yalin Heleb Hagi Ad Boker” – “the fats of My festival sacrifice shall not ‘sleep’ until morning.” This command forbids neglecting to place on the altar the parts of the sacrifice which are to be burned on the altar, throughout the night. If a sacrifice was offered, and the Kohanim neglected to place the fats on the Mizbe’ah (altar) before morning, they have transgressed this prohibition. The Sefer Ha’hinuch explains that laxity of this sort reflects a lack of respect for the sacrifices, and so the Torah issued a special command forbidding the delay of the placement of the sacrificial fats on the altar.
In regard to the timing of the placement of the fats on the altar, an important distinction exists between the Korban Pesach – the special sacrifice offered during the afternoon of the 14th of Nissan – and other sacrifices. When it comes to other sacrifices, their fats are to be placed on the Mizbe’ah either the day the animal is slaughtered, or the following night, before morning. The fats of the Korban Pesach, however, are offered specifically during the afternoon; they are not permitted to be placed on the altar at night, after the onset of Yom Tob (the first night of Pesach). Halacha forbids burning on Yom Tob sacrifices which were slaughtered on a weekday, and thus the fats of the Korban Pesach need to be placed on the altar during the day, and not at night, after Yom Tob has began. The exception to this rule is a situation where the 14th of Nissan fell on Shabbat. Although the slaughtering of the Korban Pesach overrides the Shabbat prohibitions, and thus the sacrifice is slaughtered that day, the fats may not be burned on Shabbat. In such a case, Halacha allows – and mandates – burning the fats at night, after the onset of Yom Tob. Ordinarily, however, when the 14th of Nissan falls on a weekday, the fats of the Korban Pesach must be placed on the altar that day. If they are not, then the Kohanim are in violation of the prohibition of “Lo Yalin Heleb Hagi Ad Boker,” because they are unable to place the fats on the altar during the night.
A Kohen who violates this prohibition does not receive Malkut, because this command is violated through inaction, by neglecting to place the fats on the altar on time. A famous rule establishes that a violator is liable to Malkut only if he violates a prohibition by committing a forbidden act, and not if he violates a prohibition by failing to perform a required act.
It should be noted that this prohibition differs from the parallel prohibition that applies to sacrificial meat which is supposed to be eaten. Such meat may not be left over until the morning, just as the sacrificial fats which are to be burned on the altar must be placed on the altar before morning. However, if one left over sacrificial meat until the morning, he can “rectify” his violation by burning the meat, as the Torah requires when meat is left over. The fats, however, are not burned if they are left over until morning, and thus this prohibition – against neglecting to place the fats on the altar – cannot be rectified.